A ferroelectric material is a type of dielectric materials and is a material which spontaneously polarizes even in the absence of any external electric field (with electric dipoles kept aligned) and of which the polarization direction varies depending on the electric field therearound. Typical materials of the type include barium titanate BaTiO3, lead titanate PbTiO3, lead zirconate titanate Pb(Zr,Ti)O3 and the like having a perovskite structure, which are applied to ferroelectric memories, actuators and others using the spontaneous polarization characteristics and the piezoelectric characteristics that the ferroelectric material has.
In a ferroelectric material, it is known that a larger amount of displacement of each constituent element from the normal position provides more excellent ferroelectric characteristics (spontaneous polarization, etc.).
Accordingly, for obtaining excellent ferroelectric characteristics, it is preferable to attain artificial lattice displacement. For this, various studies and developments of producing a ferroelectric thin film by the use of a substrate that differs from the ferroelectric material in point of the lattice constant thereof to thereby induce artificial stress and improve spontaneous polarization through the lattice mismatch between the ferroelectric material and the substrate have been actively tried, and the results have been reported. Apart from the stress effect of the substrate mentioned above, a ferroelectric superlattice can be produced by alternately layering two or more different types of ferroelectric materials that differ in the lattice constant (for example, BaTiO3 and SrTiO3, etc.) to thereby generate a pressure through lattice strain in the in-plane direction of the thin film surface, owing to the lattice constant difference; and according to this, studies and developments of improving spontaneous polarization have been actively tried, and the results have been reported.
However, the above-mentioned ferroelectric thin film is such that the spontaneous polarization thereof has been improved by imparting stress or lattice strain to the original ferroelectric material, and it is impossible to convert a material that is not a ferroelectric material into a ferroelectric material.
For ferroelectric memory application, preferred is use of an ultrathin film for high-speed, large-volume low-voltage operation. However, when conventional ferroelectric materials of barium titanate BaTiO3, lead titanate PbTiO3, lead zirconate titanate Pb(Zr,Ti)O3 or the like are thinned to a thickness of 50 nm, then the spontaneous polarization and the relative dielectric constant thereof may lower and they could not function as ferroelectric materials, or that is, conventional ferroelectric materials involve such an essential problem of “size effect”. Consequently, so far as conventional ferroelectric materials are used, it is difficult to provide a ferroelectric thin film having a thickness of not more than 50 nm.